The electrical activity of the brain can be examined by means of electroencephalography (EEG). Changes in the membrane stresses of neurons of the brain can be recorded by electrodes attached to the patient's scalp. When changes take place simultaneously in several neurons, the voltage fluctuations in single neurons can be summed and a measurable EEG signal can be produced. In practice, a signal recorded from the scalp originates from simultaneous changes in the post-synaptic potentials of the pyramidal neurons of the cortex. The voltage fluctuation in an EEG can be typically 5-250 μV and the frequency range can be 1-70 Hz.
EEG examinations can be used, for example, in specialised neurophysiological units in diagnosing epilepsy. There, the EEG recording can be carried out by specialist staff with high-level training and experience in using measuring devices and in the correct placement of the electrodes. However, EEG recordings could also be useful outside units specializing in EEG, for example, in an intensive care, paramedic care, and health centers, where by means of recordings could detect various disorders in the electrical activity of the brain relating to, for example, severe brain damage, cerebral infarction, cerebral haemorrhage, subarachnoid haemorrhage, intoxication and unclear consciousness disorders. For example, in paramedic care, the electrical activity of the heart (ECG) is recorded routinely, but the electrical activity of the brain, the EEG, is rarely recorded. The reason for this could be presumably the clumsy EEG sensor applications on the market, which can be slow and difficult to correctly place on the patient without specialised training and extensive experience. The lack of EEG monitoring currently presents a central diagnostic challenge, for example, in emergencies and in paramedic care, often hardly anything is known about the patient's brain activity or possible damage to the brain until the patient may have been moved to a hospital and to EEG monitoring. It would be desirable to be able to diagnose a dysfunction in the electrical activity of the brain as soon as possible and quickly start the appropriate treatment.
Currently, the EEG is most commonly measured by using an internationally standardised, so-called 10-20 system. As electrodes can be most often used cup electrodes made of, for example, silver-silver chloride (Ag—AgCl), silver, tin, gold or platinum. The 10-20 system can have several disadvantages, for example, the system uses 21 different electrodes, the positions of which in relation to the skull should be determined accurately, in order to be able to make a reliable diagnosis. A measuring wire connected to each electrode makes the measuring connection rigid and uncomfortable, hindering the patient's normal movements, and can cause interferences (movement artifacts) in the measuring signal and thus complicate the interpretation of the EEGs. Placing cup-like electrodes on the scalp can require preparation of the skin, that is, mechanical scraping of the skin to remove the dead surface layer (epidermis), and dosing of a conductive medium (electrode gel). Finally, the adhesion of the electrodes can be ensured by different attachment systems, such as tapes, bands, nets, caps or adhesive fixing paste. Thus, experience and special knowledge can be needed for measurement preparation. In addition, known equipment uses a considerable amount of time (for example, 30-50 minutes), thus delaying the initiation of the patient's proper treatment and considerably increasing the treatment costs.
Many of the current clinically used electrodes may not be compatible with magnetic imaging (MRI) and computed tomography (CT) equipment, and thus the electrodes have to be removed from the patient to help ensure safety and imaging quality. Removing and reattaching the electrodes can cause a long time-wise interruption in the EEG recording, in which case a clinically significant abnormality in the EEG can be undetected, leading in the worst case to an incorrect diagnosis. The removal and reattachment of the electrodes also can cause the patient to experience skin irritation, pain and a potential risk of skin infections.
Commercially (for example, in patent publication EP0951233 B1) available disposable electrodes, which can be adhered to the forehead, which can have only a few, for example, one to four (1-4), measuring channels and are mainly used for determining the depth of anaesthesia during operations. Due to the small number of electrodes, such solutions can be, however, unsuitable for diagnosing disorders in brain activity, for example, epilepsy, coma, cerebral hemorrhages.
One known solution is a quick-to-use matrix electrode (StatNet™, HydroDot Inc., WO2009/061920 A1), which consists of two strips placed crosswise over the head. The strips have a sandwich structure with silver-silver chloride electrodes and silver signal transmission lines integrated in flexible plastic film. In each strip, the transmission lines end on the edge of the strip, from where the signals measured can be led by means of a quick coupling to an amplifier. The strips can be coated with an adhesive by means of which the sensor remains adhered to the skin by itself. On top of the electrodes can be a porous pre-moistened pad construction. No preparation of the skin or other pre-treatment can be thus needed. The set-up time of the sensor can be, for example, 5 minutes, and the operating time of the electrodes can be, for example, 4 hours. StatNet's sensor, for example, is placed over the hair and is structurally relatively rigid (bends only in the direction of the strip), conforms poorly to surfaces curving in several directions, moves easily with the hair, and uses monitoring to ensure that it has remained in place. This sensor application is also unsuitable for patients whose head or neck should not be moved or who have injuries or measuring instruments in the cranial area. The known implementation may be suitable for paramedic and intensive care unit use, but not for several other EEG examinations, such as long-term epilepsy studies or sleep studies.
U.S. Pat. No. 6,032,065 A discloses a disposable EEG matrix electrode for use in the hairless areas of the face. The substrate can be made of a non-conductive polymer, such as Mylar. In the publication, the ground electrode is located centrally, the reference electrode on the neck, and the movement of the eyes and the EMG signal from the chin can be monitored by means of the electrodes. There are only two EEG electrodes on the temples. In this known implementation, a separate medium (electro-gel) can be used to affix the electrodes on the surface of the skin and to form a proper electrode contact.
U.S. Patent publication No. 2010/0041962 A1 discloses a matrix electrode intended for EEG monitoring, which includes electrode contacts placed on the hairless areas of the face. The publication shows a sensor construction, which makes possible lateral extension of the sensor in order for the sensor construction to fit the different face sizes of patients. The mutual placement of the electrodes with respect to one another changes when attached to faces of different size. In addition, stable attachment of the sensor construction can be ensured with a separate adhesive layer. The attachment of the sensor construction can be ensured with a structure extending behind the ear.